Here's how I plan to re-build the old Fo Mi Chin [which works at a highly inconvenient length of 47 inches]. With this new/improved geometry, I ought to be able to get him down to Dynajet size [22 inches overall length].

What you do is, when you cut the tailpipe, you leave a long strip protruding out the front to penetrate the chamber. This forms the bottom face of the intake duct [slightly convex toward the duct center]. The top of the duct is a split section of tailpipe material with a flared end. This is welded onto the front end of the tailpipe so that it extends forward past the end of the projection. Finally, an angled deflector plate is welded onto the front, mostly to save the design from being called "just another Thermojet". This plate deflects the intake flow downward into the bottom front of the chamber where it belongs.

The upper part of the pressure can dome has to be carved out to accommodate the built-up intake section. This will be a tricky area to weld up after the tailpipe is shoved into place, since the steel of the dome is pretty thin. The usual starting air / fuel tube is added on, of course. I should be able to bring this in right at the 16 ounce weight point, if the welding is carefully done.

steve wrote:It looks good to me Larry, I'm confident that it will run.

BTW, what is the best way to invert/re-weld the can bottom? I tried to make a reynst like this and ended up burning too many holes in the can.

Steve -

The only [practical] way I found to do it was to use the bottom of a second can. Punch and file out the bottom of the main can, leaving the rim untouched. Cut the bottom off the second can just above the rim. Get them centered together and tack weld in about 8 places, then weld all around, rim to rim, using just a little filler rod.

Note that the drawing shows TWO rims butted together at the front end. Now you know why ;-)

Larry Cottrill wrote:The only [practical] way I found to do it was to use the bottom of a second can.

But, why make the bottom concave? What's wrong with convex? Melenric used convex bottoms on his big thermojets on purpose. They were not built from cans, but purpose-built, so he could have chosen any shape, but chose convex.

Bruno Ogorelec wrote:But, why make the bottom concave? What's wrong with convex? Melenric used convex bottoms on his big thermojets on purpose. They were not built from cans, but purpose-built, so he could have chosen any shape, but chose convex.

I just like them that way. At least it looks like something that might someday aspire to be a flight engine. For flight, you want a dome out front, not a bucket ...

steve wrote:Larry, I feel like an idiot! It all makes sense to me now. now I just have to find two more cans.

Steve -

Note that all cans are not created equal. The reason I like the Foamy(TM) shaving cream cans, besides the fact that they are reasonably short, is that the top dome is rather tall and nicely formed, more like an egg on end than a sphere. Some pressure can tops are rather blunt, and some have pretty large 'flat' zones around the 1-inch ID rim. The Foamy cans are the nicest shaped at the dome, with the smoothest transition to the rim that I've seen.

Last night I got the intake / tailpipe assembly cut and welded up. All I need to do to finish the subassembly is hammer down the front lip to form the deflector that turns the flow down into the chamber, and flare the tailpipe. The intake upper shell was split from a small piece I had that was already flared. Welding was not exactly difficult, but not exactly a beginner's project, either. It actually went better than I thought it might - once you get out on the front end where the tailpipe is notched to form the narrow 'spade', I could just melt the edges together with no added filler rod at all. The whole thing was tacked together first, of course. Pictures of parts tomorrow, welded-up assembly some time next week.

Here's a dimensioned detail drawing showing how to cut and form the intake / tailpipe assembly. I decided that it was better to just make the intake shell with an overhang and heat and hammer it down to form the end deflector rather than weld on a tiny piece of sheet metal. Otherwise, it's just the way I showed it on the earlier drawing.

The bottom corners of the intake flare have to be flattened down to meet the exhaust pipe tube where you weld them in. Nothing tricky about it. I found that the split half shell has to be pinched in the vice to have the right fit on the tailpipe tube. This is shown on the section in the drawing. Again, nothing difficult about getting it right. As always with this tubing, the welding flame has to be kept quite small and carefully manipulated to do a smooth job. It came out looking pretty nice, really, for only a couple hours of hacking and welding, total.

I had about an hour to work on it last night, which was enough time to get the tailpipe flare and to bend the nose of the intake shell down into a deflector, as shown on the drawing. The steel split at one end where it got bent down, so I had to patch it with a tiny weld, but I'm sure the end result is smooth enough to work well.

I will probably not save the original Fo Mi Chin as an artifact, but rather cut the original tailpipe assembly off and re-use the chamber. That will save me re-creating the front end dome and plug mount. And besides that, why should I keep a pulsejet around that has to be four feet long to run? Sheesh ...

So, all that's left is salvaging the chamber off the old engine, modifying it slightly to let the new intake shell get through the rear dome, then welding it together and adding the starting air tube assembly.

With the front of the tailpipe marked and measured, the large notched-out section can be cut. The front angle of the "spade" has already been crafted with a flat file. Photo Copyright 2005 Larry Cottrill

An attempt at showing the intake area. The split shell has to be pinched significantly to get the required space. With this done, marking the tailpipe for the front notch is easy. Photo Copyright 2005 Larry Cottrill

FMCII_intake_area_crop1_small.jpg (178.82 KiB) Viewed 16012 times

The final cut that frees the split intake shell from the tube - the cut is at 45 degrees to the tube centerline, approximately. Photo Copyright 2005 Larry Cottrill

Start splitting the tube at one side of the flared end, right on the tubing seam, then bring the blade down on the opposite side. You can cut clear through like this for the first few inches. Photo Copyright 2005 Larry Cottrill

Looks very cool. i've always wanted to try that simple crescent intake. i was thinking that it will be hard to hit a certain area if you are wanting something more knowable than TLAR. When welding, you could clamp it around a bundle of the right number of welding rods , maybe.

Mike Everman wrote:Looks very cool. i've always wanted to try that simple crescent intake. i was thinking that it will be hard to hit a certain area if you are wanting something more knowable than TLAR. When welding, you could clamp it around a bundle of the right number of welding rods , maybe.

Yes, it's hard to know when you've got it. I just kept pinching the shell more and more until I was sure I must have at least the area of the 1/2-inch EMT I had used for the original Fo Mi Chin intake, since that worked fine [but was about the smallest I think I could go]. Obviously, a non-circular section is less efficient at letting air through, also. But I think that's a really sexy form for the intake to have - sort of 'quasi-concentric', if you will.

The main positive feature is that it's very low profile - not protruding belligerantly as in the Chinese or original FWE, for example. I want to get the cross-section profile as clean as possible, to make it easy to enclose an engine inside a scale model fuselage.